The importance of controlling the anti-tumor response is underscored by the observations that different types of tumors are affected differently by humoral and cell-mediated immunity. The effectiveness of an anti-tumor response is dependent on the magnitude, the rate of development, and the qualitative character of the response. The latter includes not only the proper balance between the two arms of the immune response (humoral and cell-mediated) but also the proper balance within each. The aim is to establish what constitutes an effective anti-tumor response and to determine how to achieve it. The working hypothesis is based on the assumption that the concomitant injection of antigen and antibody can effect the immune response both qualitatively and quantitatively. In the present study, the immune response to LR, L1210/MTX and L1210 cells is under study in CD2F1 mice. The class and subclass of the antibody response to these leukemic cells is being analyzed as well as the magnitude and specificity of the T-cell response. Passive antibody plus leukemic cells (or soluble antigen) are injected concomitantly to determine if and how the active immune response is modified. The quantity and quality (specificity and type) of the passive antibody injected is varied in an attempt to skew the active immune response in the direction of increased effectiveness.